I’m a huge fan of multi-speed transfer cases and crawl boxes, although to be honest, I have never owned either. They are a smart choice for those that like to own one versatile vehicle that excels at many different kinds of off-road use. Sand dune and mud driving is typically much easier with a 2:1 to 3:1 low range gear ratio. This provides the gearing to get the engine wound up into the sweet spot, yet still allows enough wheelspeed to get where you wanna go. If all you hit is rocks and slow technical trails then a 4:1 or deeper is the way to go.
I don’t need a super crawly T-case for the type of off-road driving that I do (on the farm or on old mining roads in Colorado). I’m perfectly happy with a 2:1 ratio as found in the NP205. It hasn’t let me down yet. However, if I did have a need for lower gearing, I’d probably go with Offroad Design’s NP203/205 Doubler kit, which would offer the option of a conservative 4:1 ratio if I needed it.
With the understanding that I like my rigs on the more powerful side, I am not a huge fan of overly low transfer case gearing. For the wheeling that I mostly do, a 2.72:1 or 3.0:1 works extremely well. There is plenty of mud around my neck of the woods, as there are loose hillclimbs. If it’s dry, many of the obstacles you can crawl right up. If it’s wet, you need wheelspeed.
The NP231 and NV241 are both excellent chain-driven T-cases, but both need to be fitted with a skid or protection if you are playing in the rocks. Every Dana 300 I have ever been around has leaked, which drives me crazy, but they are very versatile. If you have the money, an Atlas two-speed transfer case is a great investment and typically the only drawback you’ll encounter is a bit of gear noise.
Funny thing is that I have certain axle ratios I just like. These include 4.10, 5.13, 5.38, and 6.72. There is nothing rational about it in some cases, and in others it’s a financial, strength, or availability decision. For example, if I’m running Rockwell axles I’m sticking with the 6.72 gears. Changing them out for a custom one-off ratio is just too expensive. In a trail-specific Dana 44 the deepest I’ll go is with a 5.38. A 5.89 is available, but since my first Jeep had 5.38s that’s what I think that axle should have. Plus, the 5.38 has one extra tooth on the pinion making it marginally stronger. My Ford 9-inch axles are typically backed with 37- to 38-inch tires so they get either 4.10 or 5.13 gears depending on the application. Anything in-between has never been an option. I suppose maybe it’s because I’ve successfully run these axle ratios in 90 percent (or better) of the rigs I have owned and wheeled hard.
When I fit larger tires onto a rig I use a differential gear ratio chart to choose the new axle gear ratio. I’m not big on experimenting with differential ratios because it’s not inexpensive and if I make a mistake I’m stuck with it. Personally, I like to keep the engine rpm’s as close to stock as possible to keep the engine and transmission happy. This has worked well for me so far, as I’ve had regeared 4x4s that have performed well and didn’t suffer from a drastic mpg drop.
I tend to run 37s on most of my rigs these days and have found that a 5.13:1 ratio puts everything in a happy balance. What type of axles you have and how much power output your engine is making are two very big details to consider. There are numerous gear conversion charts and most are pretty accurate. I tend to run a little higher of a numerical gear than most since most of my wheelers are not daily-driven. For pulling out good fuel economy you have to balance where your engine makes power and how high of an rpm are you willing to live with. For the most part, differential gears are the most important gears that you can swap in. Fitting the right gear ratio inside of your diffs will prolong the life of your transmission and give the rig back the power and drivability that the larger tire set removed.
I guess I’m gonna go with the one that was the most fun for me to drive everywhere. This particular 4x4 had a 350hp V-8, a TH350 auto tranny, a Dana 300 T-case with the stock 2.62:1 low range, Dana 44 and Ford 9-inch axles with 5.13 axle gears, and 38-inch tires. It wasn’t perfect and I would smoke the transmission and grenade front axles in the rocks if I wasn’t careful, but it was just a good, fun, light-weight, inexpensive, all-around beater combo. Ideally, I would have added something like a 2:1 crawl box or doubler to compound the gearing in the rocks and save the automatic transmission. Ultimately, the best combo is really dependent on the kind of wheeling you do, your 4x4s wheelbase, weight, tire size, your driving style, and more.
Duramax turbodiesel, Allison 1000, NP205 (or any other tough T-case with a conservative low range), and the appropriate differential gearset (per the chart, and preferably spinning in AAM axles). That’s a more than suitable drivetrain combo for me whether I’m plowing snow, pulling a horse trailer through the muck, or cresting Imogene Pass in Colorado.
My dream is simple. GM 6.0L V-8 with a little tuning and air flow adjustments, 4L65E nicely built if I am going to keep it highway-friendly or a fully-built TH350 if I am trail only, two-speed Atlas with a 3.0:1 ratio, and a set of 5.13 gears. I’m also partial to how I originally built my ’99 Dodge Durango which had a factory 5.9L V-8, 46RE, NP242D, and set of 2½-ton Rockwell axles with the factory 6.72:1 gears.